This invention generally is directed at the removal of halogenated compounds. More specifically, this invention is directed at the destruction of polyhalogenated compounds by electrolysis.
Polyhalogenated compounds, such as polychlorinated biphenyls (PCBs), have been widely used in industry for a variety of applications, including insulating fluids in electrical transformers and capacitors, and as pesticides. However, these polyhalogenated compounds, particularly polychlorinated compounds, have been found difficult to destroy by conventional methods such as incineration because of high costs, incomplete combustion and the possibility that the chlorine liberated will react with water to form hydrochloric acid. Disposal of these compounds in landfill is not preferred because of the toxicity and long-term stability of these compounds. Other methods of destruction of these compounds, such as replacement of the chlorine present by other halogens, such as fluorine, are not preferred because the compounds produced also may be hazardous. Moreover, replacement of chlorine by fluorine may necessitate the use of relatively expensive materials of construction. Considerable work has been disclosed in the literature on the destruction of chlorinated compounds, including polychlorinated compounds.
European Patent Application No. 27,745 discloses a process for the electrochemical degradation of chlorinated compounds. The chlorinated compounds are placed in an electrolyte solution. An electrical charge is passed through the solution causing the chlorinated compounds to react with other molecules to form larger chlorinated molecules having a lower oxidation potential in the case of anodic oxidation, or a lower reduction potential in the case of cathodic reduction than that of the original chlorinated compounds. A divided cell preferably is used to prevent migration of the stable intermediates back to the opposite electrode. The solvent used may be aqueous, with supporting electrolyte added where needed, and at least one compound which reacts with the cation radicals formed to form compounds having a lower oxidation or reduction potential than the original compound. The non-sacrificial electrodes utilized may include carbon, platinum and steel. Preferred solutions comprise sulfuric acid and water. Solutions are disclosed in which acetonitrile, acetonitrile/water with sodium perchlorate added, and solutions of ethanol with sodium perchlorate or sulfuric acid are added for the degradation of chlorinated compounds, including polychlorinated biphenyls (PCBs). However, this process is not favored because the chlorinated compounds are not decomposed to non-toxic compounds but, rather, only reacted to form higher molecular weight chlorinated molecules. In addition, the process described in this patent requires that anodic oxidation potential and cathodic-reduction potential of the system (i.e. reactants in the cell) be smaller than the discharge potential of the solvent component, or solvent/reactive component, of the aqueous system. This, in effect, may require that the electropotential of the system be measured for each material that is to be reacted before and during the process. Failing to do so may result in the reformation of the starting material. Moreover, the relatively low electrochemical potential will result in a relatively low rate of reaction.
Japanese Kokai J50081965 teaches the electrolytic conversion of PCBs to their fluorinated analogs in the presence of calcium fluoride. The resulting fluorinated compounds are removed by adsorption. However, this process is not favored, since the resulting fluorinated compounds may be hazardous and the replacement of chlorine by fluorine is slow and does not go to completion.
U.S. Pat. Nos. 4,072,596 and 4,131,526 disclose apparatus for removing compounds which are difficult to oxidize, such as PCBs, by the use of a vessel containing stacked beds having the characteristics of alternating high and low electrical conductivity with separately controlled pairs of electrodes for the two alternate types of beds. An aqueous solution containing the contaminants passes through the successive beds and is subjected to an alternating current field for decomposition of the contaminants in the high resistivity beds, and oxidation in the lower resistivity beds by hydrogen peroxide produced in situ by the alternating current electrolysis of water. In Example III of the '596 patent the concentration of PCBs in an aqueous solution was reduced by passing the solution through the previously described apparatus and applying 29 volts, 12 amps to the electrodes of one series of beds, and 10 volts, 15 amps to the other series of beds. This process is not particularly useful, since it is limited to aqueous system having very low concentrations of PCBs.
Accordingly, it is desirable to provide a process for the decomposition of halogenated compounds, such as polychlorinated biphenyls, and aliphatic halogenated pesticides.
It also is desirable to provide a process in which the chlorine is reacted to form a relatively insoluble, easily disposable compound.
It also is advantageous to provide a process which can be utilized for the decomposition of a wide variety of chlorinated organic compounds without measuring the electrochemical potential of each compound prior to decomposition.
It would be a further advantage to utilize a process which decomposes the chlorinated organic compounds quickly in a relatively simple electrolysis cell without the use of expensive electrodes or catalysts and without the production of hazardous by-products.
The present invention is directed at a method for electrochemically removing halogens, such as chlorine, from hydrocarbons, particularly aromatics, at the anode of an electrochemical cell. The present invention is directed at the use of a sacrificial anode and an electrolyte solution.